1. Shopping like a snob. Some of the cheapest moisturizing ingredients—petrolatum and the humectants urea and glycerin—are very effective at helping the skin retain water. Surprisingly, though, researchers have also found that they actually repair compromised cells in the stratum corneum, making them even more beneficial than scientists had thought. "We used to think those ingredients simply drew moisture into the skin. Now we know they play a critical role in sending messages that affect and improve how well the skin works," says Peter M. Elias, a professor of dermatology at the University of California, San Francisco, and the author of Skin Barrier (Taylor & Francis).

2. Applying it dry. Moisturizer needs to be applied when the skin is still slightly damp from cleaning, before the water has evaporated, says Howard Sobel, a dermatologist and dermatologic surgeon at Lenox Hill Hospital in New York City. For extremely dry skin, he suggests trying a wet wrap: "Place a thin, wet cotton cloth over the skin you've moisturized, then follow with a dry one [on top]." Leave it on for 15 minutes to an hour, and do this no more than once a week. You will hear your skin make that sucking sound you make when taking the last slurp of milkshake. (OK, maybe you won't, but you'll imagine you do.)

3. Skipping it because of zits. There is mounting evidence that acne-prone skin is deficient in ceramides, a family of lipid molecules, says Ava Shamban, an assistant clinical professor of dermatology at UCLA and the author of Heal Your Skin (Wiley). She adds, "Once the barrier of the skin has been repeatedly breached with pimples, there is a disruption in the skin's ability to retain water." Also, applying moisturizer before acne medication can be helpful: "Studies have shown that using a moisturizer in combination with a topical retinoid can reduce irritation from the retinoids themselves," says Joshua Zeichner, the director of cosmetic and clinical research in dermatology at Mount Sinai Medical Center in New York City. Acne patients with oily skin should use an oil-free moisturizer, but "if you have acne and dry skin, you can certainly use a traditional moisturizer that's not oil-free," he adds.